1. Field of the Invention
The present invention relates to processing to suppress progress of oxidation of a reference electrode performed on a gas sensor and, in particular, on a sensor element for use in the gas sensor.
2. Description of the Background Art
Various gas sensors have been used to obtain the concentration of a desired gas component in a measurement gas. For example, as a device for measuring a NOx concentration in a measurement gas such as a combustion gas, a NOx sensor including a sensor element formed of an oxygen ion conductive solid electrolyte, such as zirconia (ZrO2), is known (see, for example, Japanese Patent Application Laid-Open No. 2006-284223 and Japanese Patent No. 3537983).
A sensor element of a gas sensor, including the NOx sensors disclosed in Japanese Patent Application Laid-Open No. 2006-284223 and Japanese Patent No. 3537983, obtains the concentration of a measurement-target gas component (target component) by making use of the fact that, when a measurement electrode decomposes the measurement-target gas component by its catalytic activity, the amount of oxygen ions generated at the time becomes proportional to current flowing between the measurement electrode and a reference electrode. Specifically, the relationship (sensitivity characteristics or concentration profiles) between a concentration value and a current value (output signal value) in individual sensor elements is obtained in advance with use of a mixed gas containing a target component of a known concentration, and, when the sensor elements are actually used, a measured current value is converted into a concentration value based on the sensitivity characteristics to obtain the concentration value of the target component.
When a gas sensor as described above is used, processing called pump reference processing may be performed. In the pump reference processing, oxygen in an atmospheric gas existing around the gas sensor (e.g., an exhaust gas in a case where the gas sensor is mounted on an exhaust pipe of an automobile for use) is partially supplied from an outer pumping electrode to a reference electrode through use of an oxygen pumping function of an oxygen ion conductive solid electrolyte, so that an oxygen concentration in the vicinity of a surface of the reference electrode is kept constant.
The pump reference processing is generally processing of supplying oxygen to the vicinity of the surface of the reference electrode by connecting the reference electrode to a positive terminal of an external DC power supply, connecting the outer pumping electrode, which is originally used to pump oxygen out of the sensor element, to a negative terminal of the external DC power supply, and applying a DC voltage across the outer pumping electrode and the reference electrode. More specifically, when the voltage is applied across the electrodes by the external DC power supply, in the outer pumping electrode, oxygen is ionized upon reception of electrons and taken into the sensor element, whereas, in the reference electrode, oxygen ions are oxidized to oxygen upon release of electrons, and the oxygen remains in the vicinity of the surface of the reference electrode.
In a case where the gas sensor is continuously or intermittently used, however, problems such as an increase in electrical impedance (resistance) between the outer pumping electrode and the reference electrode over time as well as a large deviation, from an assumed value (control reference value), of electromotive force generated between the measurement electrode and the reference electrode or at another location during operation of the sensor element, and poor responsiveness of the sensor element have been confirmed. It is confirmed that these events are likely to occur when the pump reference processing is performed for a long time or frequently.
Simple omission of the pump reference processing itself, however, is not necessarily effective in terms of stabilization of the oxygen concentration in the vicinity of the reference electrode. The reference electrode is used as a reference when the concentration of the measurement-target gas component is obtained (more specifically, a reference when oxygen pumping current required to calculate the concentration is obtained), and, in this regard, performing the pump reference processing is sufficiently meaningful.